Center clamped bushing transformer

ABSTRACT

A bushing current transformer is formed by mounting a pair of terminal bushings upon a tank with their axial conducting rods in laterally spaced side-by-side relation and extending into the tank, the proximate internal ends of the rods being bridged by a conductor to form a primary loop. A compression member between the tank wall and the bridging conductor applies a tensile stress to both bushing rods thereby to clamp each bushing assembly in position without the use of insulating sleeves internally of the tank.

United States Patent Donald J. Johnston Pittslield, Mass.

Nov. 4, 1969 Mar. 23, 1971 General Electric Company Inventor Appl. No.Filed Patented Assignee CENTER CLAMPED BUSHING TRANSFORMER 5 Claims, 1Drawing Fig.

U.S. Cl 336/92,

Int. Cl ..H0lf 27/02, H0lf27/28 Field of Search l74/(CT), 18, 31;336/90, 92, 173, 174, 175; 200/150 (Cursory) [56] References CitedUNITED STATES PATENTS 1,899,658 2/1933 Ainsworth l74/(C.T)UX 2,849,6948/1958 Prince 336/174 3,071,672 l/l963 Mitchell l74/(C.T)UX

Primary Examiner-Thomas J. Kozrna Att0rneysJ. Wesley Haubner, F rank L.Neuhauser, Oscar B.

Waddell and Joseph B. Forman .ABSTRACT: A bushing current transformer isformed by mounting a pair of terminal bushings upon a tank with theiraxial conducting rods in laterally spaced side-by-side relation andextending into the tank, the proximate internal ends of the rods beingbridged by a conductor to form a primary loop. A compression memberbetween the tank wall and the bridging conductor applies a tensilestress to both bushing rods thereby to clamp each bushing assembly inposition without the use of insulating sleeves internally of the tank.

PAIENTEU m2 319m LEA tall CLAMPED BUSEHNG TRANSFORMER BACKGROUND ANDOBJECTS OF THE INVENTION This invention relates to bushing transformersin generaland in particular to a high voltage bushing currenttransformer having a plurality of secondary coils.

Current transformers are commonly constructed utilizing a singleinsulating bushing housing the primary and secondary coils. in suchtransformers the primary conductor is ordinarily of eyeboltconfiguration and the secondary coils encircle and pass through thecircular loop or eye of the primary conductor. One drawback of such aconstruction is that the number of secondary coils which can beeffectively utilized is limited due to the limited available spacewithin the single bushing and the eye of the primary conductor.

Accordingly, in the manufacture of current transformers comprising aplurality of secondaries the approach that has been taken is to mount apair of bushings upon a tank in laterally spaced relation with one endof each bushing extending into the tank, and electrically bridging theinternal ends of the bushing conductors thereby forming a single primaryloop. With such construction numerous secondary coils can be disposedabout the primary conductor due to the ample space provided within thetank.

Owing to the high voltages generally impressed on the primary of acurrent transformer, the bushings commonly utilized to form the primaryloop are of oil filled type. Such bushings ordinarily comprise an axialcentral conductor or rod having threaded end portions, an insulatingcore surrounding the central conductor and at least two tubularporcelain shells in end-to-end relation enclosing the core and rod onopposite sides of an axially intermediate ground sleeve, an annular oilspace being provided between the core and shells. An oil expansionchamber is commonly provided at one end of the bushing. A mountingflange is provided on the ground sleeve for attaching the bushing to atank. Sealing gaskets are provided interposed between the porcelainshells and the ground sleeve.

In order to form a unitary insulating structure the porcelain shells,gaskets and mounting ring are held together by what is known in the artas center clamping'One manner of providing center clamping is throughthe use of two clamping members or nuts adjustably mounted upon oppositeends of the axial conducting rod thereby to hold the porcelain-shells,gaskets and ground sleeve in clamped relation therebetween. Upon thetightening of the clamping nuts the axial or central conductors areplaced in tension and the porcelain shells, gaskets and ground sleeveare placed in compression.

Another manner of axially clamping the bushing parts in assembledrelation is to utilize a porcelain shell having a large inside diameter.This shell enclosed not only the central conductor and core but also anumber of longitudinal insulating tension rods disposed parallel to theaxial conductor and attached to a clamping member at each end of thebushing, so that the porcelain shells and gaskets therebetween areplaced in compression upon tensioning of the insulating rods.

Either of the aforementioned manners of applying the axial clampingforce leaves something to be desired in applications where electricalconsiderations do not require an insulating outer sleeve at both sidesof the bushing ground ring. In the former manner, duplicate porcelainshells are necessary for mechanical reasons, whereas in the later mannerinsulating tension rods are required. Further, both of these prior artarrangements apply the axial or center clamping force to the bushingsindividually so that each bushing is a complete selfsustaining unit.

it is a principal object of this invention to provide, in a bushingcurrent transformer comprising a pair of bushings mounted upon andextending into a common tank of dielectric fluid, clamping means for thebushing assemblies operable in the absence of rigid internal insulatingsleeves.

It is a further object of this invention to provide bushing type currenttransformers of reduced cost and simplified structure utilizing a commonclamping means for all bushing parts.

SUMMARY OF THE INVENTION In carrying out the invention in one form apair of oil-filled bushings are attached by their mounting rings to awall portion of a tank containing a dielectric fluid, such as oil. Thebushings are mounted transversely to the tank wall and in laterallyspaced side-by-side relation so that one end of each axial bushingconductor or rod extends into the tank. An electrically conductivemember inside the tank bridges the proximate end portions of the axialbushing conductors thereby to form a single loop primary. Disposed aboutthe primary conductors within the tank are a plurality of secondarycoils. A compression member or column of insulating material iscompressed between the tank top and the bridging conductor whereby theforce exerted by the compression member causes both axial bushingconductors to be jointly placed in tension. A clamping member isconnected to the upper end of each axial bushing conductor with theporcelain shell and gaskets being embraced between the clamping memberand the mounting ring so that the application of a tensile force on theaxial bushing conductors places the porcelain shells and gaskets incompression.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be betterunderstood and its various objects and advantages will be m ore fullyappreciated from the following description taken in conjunction with theaccompanying drawing in which:

FIG. 1 is a side elevational view, partly in section, ofa bushingcurrent transformer embodying my invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1, an identicalpair of high voltage terminal bushings 1 and 19 are attached by theirmounting rings 2 to the top wall, or cover of a current transformer tank3. The bushings l and 19 are each formed to provide an internal annularspace for a dielectric fluid such as insulating oil, and are so mountedthat these spaces communicate with the interior of the tank 3. The tankand bushings are filled to a level near the top of the bushings with acommon body of oil 3a. The bushings are disposed in a laterallyspaced-apart or side-byside relation to each other. While the elongatebushings 1 and 19 are shown in parallel spaced-apart relation and normalto the tank cover it is within the scope of this invention to disposethe bushings with their axes in any desired angularly displaced relationwhich is appreciably removed from a position of axial alignment. Thereason for such a limitation will become apparent from a laterdiscussion of the manner of providing central clamping.

Since the bushings are identical only one of the bushings will bedescribed. Bushing l, for example, consists of a porcelain shell 4surrounding an insulating core 5 which may be oil impregnated paper. Theouter shell can be of a unitary construction, as shown, or can be madeup of a number of stacked shells with gaskets therebetween. Theporcelain shell 4 is stacked on top a metallic mounting ring 2.interposed between the porcelain shell and the mounting ring 2 is asealing gasket 6. A transparent glass cylinder I1 is mounted as an upperextension of the insulating shell 4 and the bushing is closed at the topby a cap 10. Disposed inside the shell 4 and the core 5 and running theentire length of the bushingis an axial bushing conductor 7, each end ofwhich is threaded. At the upper end of each bushing is a clamping memberor stop nut 8 which is screwed to conductor rod 7 to apply clampingpressure to the cap llll in a manner to be more fully describedhereinafter. As is well known to those skilled in the art, a springassembly (not shown) may be provided within the cap 10 to maintainpressure on the clamped assembly.

Between the insulating core 5 and the insulating shell 4, 11, an annularspace 9 is provided, and the space 9 is filled with the oil 38a to alevel within the glass extension ill. The space 9 above the oil levelprovides an expansion chamber into which the oil may rise when itbecomes heated during operation of the transformer. The glass shell 11allows visual inspection of the oil level in the bushing assembly.

The lower end portions of the inner core and the axial bushing conductor7 pass through an opening 12 and into the tank El. Surrounding thisportion of the core 5 and axial bushing conductor 7 a number oftransformer secondary coils 13 are provided. Although only two coils l3are shown surrounding each core 5 and axial bushing conductor 7, it isof course contemplated that if desired a greater number of coils may beutilized. The coils l3 are supported by cup-shaped saddle 14 which ispreferably formed of insulating material and suspended from the interiorupper wall of tank 3. it should be noted that although the secondarycoils 13 are shown disposed inside the tank, it is contemplated that ifdesired the coils 13 may surround the bushing conductors outside of thetank.

Electrically connecting the proximate lower ends 15 of the axial bushingconductors 7 inside the tank 3 is a bridging conductor lid which isattached to each rod 7 by a stop nut 17. The serial electrical path madeup by one axial bushing conductor 7, the bridge member 16, and the otheraxial bushing conductor 7 forms a lJ-shaped primary circuit of thecurrent transformer.

Within the tank 3 and between the bushings an insulating compressionmember or column 18 is interposed and compressed between the top of tank3 and the bridge conductor in. The effect of such compression is toapply a downward force on the bridge member 16, which force by workingagainst nuts l7 places the axial bushing conductors 7 in tension. Withtension on the axial bushing conductors 7, a downward force will beimposed on the top clamping nut 8 and cap member ill thereby compressingthe gasket glass shell ill, the porcelain shell 4 and the gasket 6against the mounting ring l2.

Thus, according to my invention, the central clamping force is appliedto the bushings jointly by the interaction of the stop nuts 17, theconducting bridge lit), the axial bushing conductors 7 and the top stopnuts As shown, the bushings l and l9 are disposed parallel to each otherand normal to the conducting bridge 16. Although this is the mostexpeditious geometry nevertheless various other geometric arrangementsare contemplated. it should be appreciated that since the compressionmember 18 applies a force to the bridge member l6 which in turn appliesthat force to the axial bushing conductors 7, the maximum force will beapplied to the axial bushing conductors when the force applied to themby the bridge member has only one component and that component isparallel to the axes of the axial bushing conductors. This conditionexists when the bushings are parallei to each other and normal to thebridge member. If, however, the bushings are disposed at an angle toeach other and hence at an angle to the direction of the force exertedby the compression member 113, then only that component of the forcewhich is parallel to the axes of the axial bushing conductors 7 isavailable to place them in tension. This component is necessarily lessthan the total force exerted by the compression member ill.

in practicing my invention, the bushings should be disposed relative tothe direction of the force exerted by the compression member such thatthere will be a substantial component of that force parallel to thecentral conductors available to place the axial bushing conductors intension. This condition will occur when the bushings are disposed in alaterally spaced-apart or side-by-side arrangement. By this arrangementll mean to include all bushing arrangements from the preferred paralleldisposition to an angular disposition wherein the bushings areappreciably out of mutual axial alignment.

As was heretofore noted when the bushing axes are parallel to each otherand to the direction of the force exerted by the compression member l8the tensile force on the axial bushing conductors will be at itsmaximum. As the angular disposition of the bushings increases toward acondition of axial align ment the tensile component of force on thecentral conductors decreases. For wide angular dispositions it may beexpeditious to utilize a pliable flexible bridge member 16, such as abraided strap conductor or the like, in order to maximize the tensileforce exerted on the central conductors.

In accordance with my invention, by eliminating the need for lowerporcelain shells and insulating tension rods the manufacture of abushing transformer is simplified thereby improving structuralreliability while decreasing manufacturing costs.

Further, in my improved transformer structure, there is nothing toimpede the free flow of oil between the bushings and the tank, therebypermitting the bushings to operate somewhat cooler than in prior artdevices.

While a particular embodiment of the invention has been shown anddescribed, it will be obvious to those skilled in the art that variouschanges and modification may be made without departing from theinvention in its broader aspects. I therefore intend herein to cover allsuch changes and modifications as fall within the true scope and spiritof my invention.

1 claim:

1. A bushing current transformer comprising:

a. a tank having a wall portion;

b. at least two terminal bushings attached to said wall portion in alaterally spaced-apart relationship, each of said bushings comprising aninsulating shell and an axial conductor extending longitudinallytherethrough in spacedapart relation, one end portion of each said axialconductor being disposed inside said tank;

c. a common body of dielectric fluid filling said tank and at leastpartially filling said bushings;

d. a secondary coil disposed around one of said axial conductors in theregion of said tank; and

e. means for imposing a central clamping force on each of said bushingscomprising:

i. an electrically conductive bridging member connecting the proximateends of said axial conductors within said tank;

ii. an insulating compression member interposed between said wallportion and said bridging member at a point intermediate said axialconductors; and

iii. stop means located at the upper ends of said central conductors;whereby a force exerted by said compression member acting through saidbridging means to said axial conductors produces a substantial tensilecomponent of force on said axial conductors and thereby produces acompression force on said insulating shells.

2. A bushing current transformer as specified in claim 1 wherein thebushings are disposed in a generally parallel relationship to eachother.

3. A bushing current transformer as specified in claim ll wherein saidbridging member is rigid.

4. A bushing current transformer as specified in claim l wherein saidbridging member is flexible,

5. A bushing current transformer comprising:

a. a tank containing oil, said tank having a top wall portion;

b. a parallel pair of oil-filled bushings attached to said tank wall, influid communication with said tank, said bushings each comprising amounting ring, an insulating shell, an insulating core and an axialconductor extending longitudinally thereof, a portion of said insulatingcore and a portion of said axial conductor extending into said tank;

0. a secondary coil disposed around one of said axial conductors in theregion of said tank;

d. common means for imposing a joint central clamping force on each ofsaid bushings comprising:

i. a rigid electrically conductive bridge member connecting the ends ofsaid axial conductors extending into said tank;

ii. an insulating member compressed between said top portion of saidtank and said bridge member at a point intermediate said axialconductors, and

means to said axial conductors produces a tensile force in said axialconductors and thereby causes the insulating shells to be placed incompression.

73 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,571, 773 Dated March 23, 197].

Inventor(s) Donald L. Johnston It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Front page, line 1 Change the inventor's name from "Donald J. Johnston"to Donald L. Jbhns" Signed and sealed this 13th day of July 1971 (SEAL)Attest:

EDWARD M.FLETGHER,JB. WILLIAM E. SCHUYLER, JR. Attesting OfficerCommissioner of Patents

1. A bushing current transformer comprising: a. a tank having a wallportion; b. at least two terminal bushings attached to said wall portionin a laterally spaced-apart relationship, each of said bushingscomprising an insulating shell and an axial conductor extendinglongitudinally therethrough in spaced-apart relation, one end portion ofeach said axial conductor being disposed inside said tank; c. a commonbody of dielectric fluid filling said tank and at least partiallyfilling said bushings; d. a secondary coil disposed around one of saidaxial conductors in the region of said tank; and e. means for imposing acentral clamping force on each of said bushings comprising: i. anelectrically conductive bridging member connecting the proximate ends ofsaid axial conductors within said tank; ii. an insulating compressionmember interposed between said wall portion and said bridging member ata point intermediate said axial conductors; and iii. stop means locatedat the upper ends of said central conductors; whereby a force exerted bysaid compression member acting through said bridging means to said axialconductors produces a substantial tensile component of force on saidaxial conductors and thereby produces a compression force on saidinsulating shells.
 2. A bushing current transformer as specified inclaim 1 wherein the bushings are disposed in a generalLy parallelrelationship to each other.
 3. A bushing current transformer asspecified in claim 1 wherein said bridging member is rigid.
 4. A bushingcurrent transformer as specified in claim 1 wherein said bridging memberis flexible.
 5. A bushing current transformer comprising: a. a tankcontaining oil, said tank having a top wall portion; b. a parallel pairof oil-filled bushings attached to said tank wall, in fluidcommunication with said tank, said bushings each comprising a mountingring, an insulating shell, an insulating core and an axial conductorextending longitudinally thereof, a portion of said insulating core anda portion of said axial conductor extending into said tank; c. asecondary coil disposed around one of said axial conductors in theregion of said tank; d. common means for imposing a joint centralclamping force on each of said bushings comprising: i. a rigidelectrically conductive bridge member connecting the ends of said axialconductors extending into said tank; ii. an insulating member compressedbetween said top portion of said tank and said bridge member at a pointintermediate said axial conductors, and iii. clamping means located atthe upper ends of said axial conductors; whereby a force exerted by saidcompressed insulating member acting through said bridge means to saidaxial conductors produces a tensile force in said axial conductors andthereby causes the insulating shells to be placed in compression.